antialiased stroke-rects that are thin draw wrong

Issue description

if the strokeSize is < 1 in antialaising, SkScan::AntiFrameRect may draw wrongly (potentially twice on the same pixels).
SkScan::AntiFrameRect wants to draw in 3 stages:
1. skim the outer rect of the frame, computing partial coverage
2. fill in the sides/top/bottom with solid depending on how thick the stroke is
3. skim the inner rect of the frame, computing partial coverage
For thin rects, #2 is correctly skipped, but we still do #1 AND #3. We need to detect when the stroke lies wholly within 1 pixel, and just skim it once, merging the coverage of the inner and outer rect.

Here's a better fiddle that shows the current result on CPU / GPU
https://fiddle.skia.org/c/0d17450ca4fb51423d09a868548e0838
(screen captures are attached in case the behavior changes)
Of note:
- raster at .3 .4 .5 .6 .7 look the same
- gpu at .3 .4 .5 .6 .7 have same geometry but different darkness
- raster at .7 flips artifacts compared to .8
Also, the original description may be no longer relevant.
Moving to icebox, since this is probably good enough for now.